Conventionally, an automotive system includes an internal combustion engine, such as for example a compression engine or a spark ignition engine. The internal combustion engine usually includes an engine block defining at least one cylinder having a piston, and a cylinder head that closes the cylinder and cooperates with the piston to define a combustion chamber. A fuel and air mixture is disposed in the combustion chamber and ignited, resulting in hot expanding exhaust gasses causing reciprocal movements of the piston, which rotates a crankshaft.
The fuel is provided by at least one fuel injector, which may be located inside the combustion chamber. The fuel injector receives the fuel from a fuel rail, which is in fluid communication with a high-pressure fuel pump that increases the pressure of the fuel received from a fuel source (tank). More particularly, the high-pressure fuel pump may include a reciprocating plunger, which is accommodated in a cylinder communicating with an inlet and with an outlet for the fuel. The plunger is actuated by a camshaft, which is driven by the crankshaft of the internal combustion engine. During expansion strokes of the plunger, the fuel is drawn from the inlet of the pump into the cylinder. During compression strokes, the fuel contained in the cylinder is supplied at higher pressure through the outlet of the pump into the fuel rail.
A fuel-metering valve is usually associated to the high-pressure fuel pump to regulate the fuel flow-rate, which is supplied into the fuel rail. The fuel-metering valve may be integrated in the high-pressure fuel pump, in order to realize a single device that is usually referred as fuel metering unit. The fuel-metering valve may be a suction control valve (SCV) or a digital valve.
A suction control valve is generally located at the inlet of the high-pressure fuel pump and includes a valve member that is movable between a closed position, which prevents the fuel to pass through the valve, and a fully open position, which allows a maximum amount of fuel to flow towards the fuel pump. The valve member is moved by an electric actuator, typically a solenoid that converts an electrical current into a magnetic field and then into a motion of the valve member. Depending on the energizing current, the valve member can assume any positions between the closed position and the fully open position. More particularly, some embodiments provide that if no electrical current is supplied to the actuator, the valve member remains in its fully open position. Progressively increasing the electrical current supplied to the actuator, the valve member moves towards its closed position. Other embodiments provide that if no electrical current is supplied to the actuator, the valve member remains in its closed position. Progressively increasing the electrical current supplied to the actuator, the valve member moves towards its fully open position. In both cases, the suction control valve regulates the flow-rate of the fuel, which is drawn inside the pump cylinder during the expansion strokes of the pump plunger.
A digital valve is generally located in a recirculation conduit that connects the cylinder of the high-pressure fuel pump back to the fuel tank. The digital valve includes a valve member, which during the compression stroke of the pump plunger, is moved between an open position and a closed position. As long as the valve member remains open, the pump plunger shoves the fuel from the pump cylinder into the recirculation conduit and then back into the fuel tank. As soon as the valve member is closed, the pump plunger increases the pressure of the fuel within the pump cylinder and supplies it into the fuel rail. The valve member is moved by an electric actuator, which is driven by a pulsed electric signal. In this way, varying the timing of the electric pulses that form the driving signal, the valve member can be closed in different instants during the compression stroke of the pump plunger, thereby regulating the volume of fuel, which is supplied into the fuel rail.
Regardless of how they actually work, the final effect of both digital valves and suction control valves is that of regulating the average flow rate of fuel that is globally supplied by the high-pressure pump into the fuel rail, and for this reason they are all classified as fuel metering valves.
Any fuel-metering valve is typically connected to a control apparatus of the automotive system, which includes several sensors and at least an electronic control unit (ECU). In order to operate the fuel metering valve, the electronic control unit is generally configured to perform a control cycle that includes the following steps: setting a target value of the fuel pressure inside the fuel rail, for example on the basis of the engine working conditions; determining a target value of the fuel flow-rate to be supplied into the fuel rail to meet the target value of the fuel rail pressure; determining a value of the adjustable parameter of the electric signal driving the fuel metering valve, namely the electrical current (for SCVs) or the timing of the electric pulses (for digital valves), that causes the high-pressure fuel pump to supply the target value of the fuel flow-rate; and finally setting the adjustable parameter of the electric signal at that determined value. More particularly, the target value of the fuel flow-rate is generally determined as the sum of two main contributions, namely a feed-forward contribution and a feedback contribution.
The feed-forward contribution is determined by means of an open loop approach that provides for using the target value of the fuel rail pressure as input of a mathematical model of the fuel rail, which yields as output a value of the fuel flow-rate indicative of the quantity of fuel that exits the fuel rail at the target pressure value, due to the operation of the fuel injectors and their leakages. The feed-back contribution is determined by means of a closed loop approach that provides for measuring a value of the pressure inside the fuel rail, for calculating an error between this measured value and the target value of the fuel rail pressure, and for using this error as input of a PI controller, which yields as output a value of the fuel flow-rate aimed to compensate the fuel pressure error.
Once the target value of the fuel flow-rate has been calculated, the corresponding value of the adjustable parameter of the electrical signal driving the fuel metering valve is determined according to another open loop, which provides for using the target value of the fuel flow-rate as input of a correlation function that yields as output a corresponding value of said adjustable parameter.
A drawback of this approach is that the correlation function is generally a nominal function, which is provided by the supplier of the fuel metering valve and which only represents a theoretical relationship between the fuel flow-rate and the adjustable parameter of the electrical signal driving the fuel metering valve, whereas the real behavior of each single fuel metering valve may be different due to production spreads, production tolerances and many other factors such as thermal drifts.
As a consequence, for a given target value of the fuel flow-rate, the nominal correlation function generally yields a nominal value of the adjustable parameter of the electrical driving signal which differs by an offset from the value that really allows the fuel metering valve to attain the target value of the fuel flow-rate.
This offset is currently compensated by the integral term of the PI controller, which adjusts the feedback contribution of the target value of the fuel flow-rate, so that under stable conditions the fuel-metering valve allows the high-pressure fuel pump to attain the correct fuel flow-rate. However, if the value of the offset is too big, this compensation may cause instability of the closed loop.
Moreover, if the target value of the fuel flow-rate changes abruptly, for example during fast transition phases, the PI controller may be not fast enough to compensate a possible change of the offset and a big error may arise between the target value and the real value of the fuel flow-rate supplied into the fuel rail.